video editing: add mask has no menu
The first thing that you have to do is create a mask.

There are two places where you can do this:


In the Image Editor.


Or it can be created on the Movie Clip Editor 

Go to the Motion Tracking Screen Layout and load in your movie. Select Mask in the bottom options panel where it probably currently says Tracking.

 --> 


Click  to add a new mask, or select the one you already made from the dropdown. Make sure your mask is named, so that you recognize it:



Key Ctrl +  LMB to add a new control point. Keep clicking to mask your object. When done, key A once or twice until everything is selected and key Alt + C last to connect the polygon. If you want the mask to move, key I to insert a keyframe, move the time cursor in the Timeline, move the control points, and key I again. They will glide in between.

When you are done with the mask, go back to the Video Sequence Editor, and your mask should be there.